The parents of brain tumour patient Ashya King said they were "relieved" and "tired" as they left a Spanish prison and prepared to be reunited "as soon as possible" with their five-year-old son.

Brett and Naghmeh King were arrested on Saturday after they took Ashya from Southampton General Hospital without the consent of doctors last Thursday.

Brett King talks to the press as he leaves Soto Del Real prison with his wife in Madrid, Spain, Tuesday, Sept. 2, 2014

They walked free from Soto del Real prison near Madrid after British authorities dropped the case against them.

Mr King told reporters: "We're very relieved to be free."

A Spanish National Police officer stands beside a Civil Guard's van that Naghemeh and Brett, parents of seriously ill Ashya King, 5, left in, at the Spanish High Court in Madrid September 1, 2014

Speaking in both Spanish and English he said: "We are very grateful to Spain for the support and help we have received.

"We will go to see my son as soon as possible, we have been dying to see his face for so long.

Undated video grab taken from a video posted on YouTube by Naveed King showing him holding up a food package saying that his brother Ashya had enough food after he was taken by his parents from hospital without doctors' consent

"Thank you to Spain for helping us and thank you to England too."

He added: "I'm sorry we can't say more, I'm very tired."

Undated handout photos issued by Hampshire Police of Brett King and Naghemeh King, the parents of Ashya King

The Kings were released after their lawyer Juan Isidro Fernandez claimed that they had already been let out of Soto del Real prison and were on their way to Seville in southern Spain.

They were accompanied by two women as they left the jail gates and were driven off in a grey Ford Fiesta by Spanish nationals after a tired-looking Mr King spoke to reporters.

16 year old Ethan Dallas, friend of the family of brain tumour patient Ashya King, delivers a petition of over 100,000 names calling for his parents release from a Spanish jail in Madrid

Ashya's brother Naveed, asked his reaction to his parents being freed, told Sky News: "When I first heard the news, I wanted to cry, but at the same time I was so happy. We are thrilled to be able to see them, and hopefully soon we will be able go and see Ashya together."

A judge ordered the release after the Crown Prosecution Service (CPS) said it was seeking the withdrawal of the European arrest warrant that sparked the pursuit of the parents. The next hearing in the case had been likely to take place tomorrow.

The CPS said it had arranged with Southampton Magistrates Court for proceedings to be dropped and the arrest warrants discharged.

It said: "The CPS has urgently reviewed the case and we consider there is insufficient evidence for a realistic prospect of conviction for any criminal offence. We have therefore decided to stop the criminal proceedings."

Prime Minister David Cameron said: "I welcome the prosecution against Ashya King's parents being dropped. It's important this little boy gets treatment and the love of his family."

Simon Hayes, police and crime commissioner for Hampshire, said he would seek assurances about the "quality of the information" provided by the hospital to police.

He said: "In light of today's announcement by the CPS, I now need to be assured as to the quality of the information given by Southampton General Hospital to Hampshire Constabulary.

"Now that Ashya is receiving the medical assistance he requires and Mr and Mrs King will be reunited with him, this is the time to analyse decisions that were made."

University Hospital Southampton NHS Foundation Trust said the tumour was successfully removed on July 24.

It added: "After surgery, a combination of radiotherapy and chemotherapy, in accordance with agreed international protocols, is needed to prevent the tumour from returning. For best results this treatment should start within four to six weeks of surgery.

"With such treatment, Ashya's chances of survival are very good (between 70- 80%).

"During discussions, Ashya's family indicated that they wished him to undergo proton radiotherapy instead of standard radiotherapy. This option was explored with the family and they were informed that in Ashya's case there is likely to be no difference in survival between standard radiotherapy and proton radiotherapy and overall no proven significant benefit.

"Therefore, the Trust considers there is no benefit to Ashya of proton radiotherapy over standard radiotherapy.

"This view is supported by a national independent expert body.

"Despite this, the Trust agreed with the family to refer Ashya for proton radiotherapy, as the family had indicated that they could fund it privately.

"On 28 August 2014, during unsupervised leave on the Trust's grounds, Ashya's family chose to remove him without informing or seeking the consent of medical staff."

The Trust had been concerned for Ashya's safety for many reasons, and contacted the police, in line with Trust policy, to alert them to the situation.

It added: "The Trust will offer any assistance that it can to ensure that Ashya receives urgent treatment at an appropriate hospital."

Patricia King, paternal grandmother of Ashya, told Sky News: "The way everybody has behaved - the hospital who are now backtracking, the police who are now backtracking. They've done wrong, they all know they're wrong."

Health Secretary Jeremy Hunt said the Government was proposing to fly a top oncologist to Spain to advise the family.

He said: "What we want to do is make sure that Ashya's family get the best independent advice.

"We are arranging for an independent expert to fly if the family would like to Spain to give them advice as to exactly what the right course of action is for Ashya going forward."

Mr Hunt said the NHS offers proton beam therapy for children who need it and had funded 99 patients in the last year, adding: "It is not always appropriate, it is not always safe."

Danny King told Sky News that the family was hoping to get Ashya, who is in hospital in Malaga, to Prague for proton therapy. "First my parents have to get to see Ashya, and get the family together once again."